In a heat engine or heat pump, a heat exchanger may be employed to transfer heat between a thermal storage material to a working fluid for use with turbomachinery. If the heat engine is reversible, e.g., it may also be a heat pump, then the working fluid and heat exchanger may be used to transfer heat or cold to a plurality of thermal stores. The thermal energy within a given system may be stored in various forms and in a variety of containers, including pressure vessels and/or insulated vessels. For example, in solar thermal systems with storage, molten salt is commonly used to store thermal energy, while a heat exchanger is used to transfer that energy to steam or other working fluid suitable for driving turbomachinery. Thermal energy may also be stored in solids, for example in thermocline storage, where a working fluid is circulated directly over the solid material. Because the thermal storage functions as a direct-contact heat exchanger, the vessel that contains the thermal storage may be at the high inlet pressure, for example, 30 to 100 bar.
Power generation systems that utilize thermal energy storage, such as pebble-bed fission reactors and concentrated solar thermal, generally operate at high pressures and high temperatures In pebble bed fission reactors, because of the need for radiation containment, a pressure vessel containment system is normally surrounded by a secondary containment system to limit the danger of sudden pressure release. This increases cost relative to a single-containment system. In the case of concentrated solar, existing solutions commonly use multiple heat exchangers to transfer heat to high temperature and high pressure gas so that high pressure is not needed for the storage material; however, this results in inefficiency due to high approach temperatures and wasted heat.